Polylactic acid (PLA) is a thermoplastic, biodegradable aliphatic polyester that is manufactured from renewable sources by microbial fermentation, and has wide range of applications. Like most thermoplastic materials, it can be processed into a fibre or film. PLA has a wide range of applications, such as woven shirts, microwavable trays, and hot-fill applications. PLA is currently used in a number of biomedical applications, such as sutures, stents, dialysis media and drug delivery devices. In view of its biodegradability, it may also be used in the preparation of bioplastic, useful for producing loose-fill packaging, compost bags, food packaging, and disposable tableware. In the form of fibers and non-woven textiles, PLA also has many potential applications, for example as upholstery, disposable garments, awnings, feminine hygiene products, and diapers.
Bacterial fermentation is currently used to produce lactic acid from corn starch or cane sugar. However, lactic acid cannot be directly polymerised to a useful product, owing to the generation of water which degrades the forming polymer. Instead, it is dimerised to lactide. PLA of high molecular weight is produced from the lactide by ring-opening polymerization using most commonly a catalyst (e.g. tin). This mechanism does not generate additional water, and hence, a wide range of molecular weights is accessible. Typically PLA has a molecular weight number average (Mn) between 75 000 and 100 000 Dalton.
A number of microbes are capable of producing lactic acid by aerobic and anaerobic fermentation processes. Lactobacillus species are currently used extensively in industry for starch-based lactic acid production. The majority of these species lack the ability to ferment pentose sugars such as xylose and arabinose. Although Lactobacillus pentosus, Lactobacillus brevis and Lactococcus lactis are able to ferment pentoses to lactic acid, pentoses are metabolized using the phosphoketolase pathway which is inefficient for lactic acid production. Indeed, in the phosphoketolase pathway, xylulose 5-phosphate is cleaved to glyceraldehyde 3-phosphate and acetyl-phosphate. With this pathway, the maximum theoretical yield of lactic acid is limited to one per pentose (0.6 g lactic acid per g xylose) due to the loss of two carbons to acetic acid.
In most platform host organisms such as E. coli, production of lactic acid at high titers is either inefficient or toxic. The production of lactic acid at neutral pH typically results in the production of Ca-lactate, which has to be converted into lactic acid by the addition of sulphuric acid, resulting in the formation of CaSO4 (gypsum) as by product. To produce lactic acid directly, the fermentation must be executed at low pH (preferably at least one unit lower than the pKa value of lactic acid, 3.85). Lactic acid however is toxic to microorganisms, as in its protonated form it acts as an uncoupler that destroys the membrane potential. Thus, while quite some micro-organisms may be tolerant to low pH only a limited number of organisms are suitable for organic acid production in that they are tolerant to organic acids at reduced pH.
An important drawback to microbial fermentation is the cost. As several microorganisms are unable to synthesize some of the amino acids or proteins they need for growing and for metabolizing sugars efficiently, they often must be fed a somewhat complex package of nutrients, increasing the direct expense to operate the fermentation. In addition, the increased complexity of the broth makes it more difficult to recover the fermentation product in reasonably pure form, so increased operating and capital costs are incurred to recover the product. Also, the use of corn as the feedstock competes directly with the food and feed.
Accordingly, there remains a need for improved methods for PLA production which utilise micro-organisms tolerant of high levels of lactic acid in a minimal fermentation broth.